There exists in the art a storable antenna assembly, commonly called a stem antenna, which is formed of a tubular extendible element. The tubular extendible element is made of a metallic tape which is pre-stressed or formed in a direction generally transverse to its longitudinal axis. The tape is rolled onto a reel for storage and when unreeled it curls about its longitudinal axis to form an elongated tubular structure to form a generally circular or elliptical contour which is fairly rigid and self-supporting. In some of these structures two or more tapes are interlocked and/or hinged to provide different types of cross-sections. Typical types of tubular extendible elements, which are used as stem antennas are shown, for example, in U.S. Pat. Nos. 2,157,278; 3,144,104; 3,144,215 and 3,331,075.
Prior art tubular extendible elements heretofore have been used primarily in the manner of conventional radio frequency antennas, that is, as a monopole antenna element alone or as part of an array of antenna elements, to radiate or receive electromagnetic energy. In such prior art devices, the stem antennas have been oriented either vertically, or at some other angle with respect to the ground, or have been extended from space vehicles, and the elements act as conventional long wire, or short length wire type antennas.
In my application Ser. No. 707,725, filed Feb. 23, 1968 and entitled "Storable Waveguides for Electronic Systems," now U.S. Pat. No. 3,541,568, granted Nov. 17, 1970, particular types of tubular extendible elements are shown which are used as guided wave mediums for propagating electromagnetic energy along the length of the element in various modes. Antenna radiator devices, such as horns, mushrooms, etc., are attached to these elements so that the energy propagated down the guide can be radiated from the end of the extended element in a predetermined manner. The self-supporting characteristics of the tubular extendible element, which also serves as the guided wave medium, make it possible to elevate the antenna radiator device for a considerable distance above the ground perpendicularly or at some other angle with respect to the earth's surface.
In some applications there exists a need for transmitting energy which cannot be readily propagated by waveguides. For example, it is quite conventional to transmit and propagate energy below a frequency on the order of 1,000 MHz. by using what are commonly called two-wire transmission lines. These transmission lines can take several forms, such as coaxial lines, open-wire transmission lines, so-called twin-lead transmission lines commonly used in television receivers, and many other conventional types of lines. Heretofore, such lines have been "soft" in nature, that is, they can be reeled and have little or no characteristic of rigidity in the direction of their elongation. While these prior art transmission lines function satisfactorily to propagate the energy, they are not self-supporting. Therefore, they can only be used in an elevated manner if auxiliary structure is provided to support the line. For example, in a situation where an antenna radiator is to be located above the ground it is necessary to provide a tower or a mast to support the antenna and the transmission line. Also, such prior art transmission lines cannot be run readily over open spaces, such as across a ditch or stream, without an external means of support.
A problem commonly encountered with tubular extendible elements is the change of material characteristics caused by temperature. Metal usually used for the element material will change its dimensions as it is heated or warmed, if only by solar illumination. This causes problems in environments where the element has one face exposed to the sun while the other face is in shadow. Heretofore, structures have metallic extendible elements which are to be used in applications where they will be subjected to temperature gradients have had to have provisions to compensate for the change in material dimensions caused by temperature. Some compensating provisions have been complex and costly.